This invention relates to methods and apparatus for obtaining a variable packing density of hollow fibers in wound fiber cartridges such as oxygenators, and more particularly to the variation of packing density throughout the thickness of the cartridge.
Blood oxygenator cartridges are typically made by winding hollow fibers onto a core in a criss-cross pattern until the successive layers of fibers build up into a bundle of desired thickness. The core 1.5 and fiber bundle are then inserted into a cylindrical cartridge housing. The ends of the fiber bundle are potted in the housing and are then cut so as to form a multitude of individual fibers extending longitudinally through the cartridge. Oxygen is introduced into the fibers at one of their potted ends, and blood is introduced into the cartridge to flow over and around the oxygenated fibers. The fibers act as a membrane through which oxygen migrates to bond with blood cells as they contact the fibers, and to drive carbon dioxide back into the fibers.
The hollow membrane fibers used in oxygenators are very expensive and account for about 30% of the cost of the oxygenator. At an outside diameter of about 300 microns, they are also very delicate and must be wound with an essentially constant tension.
It is thus desirable to use a minimum amount of fiber in an oxygenator cartridge, but such minimization is limited by the fact that enough fiber surface must be provided to allow most or all blood cells to contact a fiber during their transit through the oxygenator. The efficiency of the oxygenator can be greatly improved by minimizing any laminar blood flow patterns through the fiber bundle, so as to cause the blood stream to mix thoroughly as it traverses the cartridge. Also, the geometry of a typical oxygenator is such that blood enters the fiber bundle more or less evenly around the circumference of the fiber bundle but exits it at a single point on its circumference. Consequently, the blood flow is not even throughout the fiber bundle.
In order to reduce the radial extent of the fiber bundle, it is customary to pack the fibers during winding by pressing a packing roller against the outside of the fiber bundle as it is being wound. The pressure of the packing roller causes the fibers of each layer to lie firmly against the fibers of the preceding layer. The amount of packing pressure determines at least in part the blood flow characteristics of the fiber bundle. Too little pressure allows the formation of laminar flow channels; too much impedes the flow of blood.
It has previously been proposed in U.S. Pat. No. Re. 36,125 to Haworth et al. that the efficiency of an oxygenator could be improved by increasing the packing fraction (i.e. the fraction of a unit volume of bundle space occupied by fiber) in successive radially outward sections of the fiber bundle. This process, however, still admits of a uniformity within each section that promotes the formation of laminar flow channels, and it does not help the uneven distribution of blood flow around the circumference of the fiber bundle.
The present invention promotes mixing of the blood in a fiber-wound oxygenator by discouraging the formation of laminar flow patterns. It also causes the blood flow to be distributed more evenly around the circumference of the fiber bundle. The invention achieves this result by varying the packing density of the fibers longitudinally and/or circumferentially of the bundle. This is preferably achieved by cyclically varying the shuttle speed or packing pressure, and/or by using packing rollers with an irregular geometry.